This invention relates to a method for upgrading tar sand bitumen for the preparation of useful hydrocarbon products therefrom, such as a higher quality syncrude essentially free of metals and asphaltenes and with a much lower molecular weight. In particular, the invention relates to a method for upgrading bitumens derived from tar sands which contain mineral matter including a fine particle size fraction and a coarser size fraction.
Extensive deposits of tar sands, bituminous sand, bituminous diatomite and similar materials are known to exist throughout the world. These materials comprise a siliceous matrix of sands, sandstones or diatomaceous earth which is coated or saturated with relatively high molecular weight hydrocarbon materials. These deposits are generally located at or near the earth's surface, although some deposits may be buried by as much as two thousand feet of overburden. It has been estimated that the reserves of petroleum products recoverable from the known deposits of tar sands would be approximately equivlant to the world-wide reserves estimated for conventional crude oil.
As mined, the tar sands are present in general as agglomerates or lumps comprising sand or diatomite, mineral particles, water and viscous hydrocarbonaceous material called bitumen. While there is no universally accepted definition of "bitumen", it may be characterized as that portion of petroleum that exists in the semi-solid or solid phase in natural deposits. It has been proposed by the United Nations Institute for Training and Research (UNITAR) that bitumens, or natural tars, be defined as the the petroleum component which has a viscosity greater than 10,000 mPa.s (cp) measured at the conditions in the deposit and gravity greater than 1,000 kg/m.sup.3 (less than 10.degree. API) at standard conditions of 15.6.degree. C. (60.degree. F.) and a pressure of one atmosphere. The definition was suggested at the Second International Conference on Heavy Crude and Tar Sands, held in Caracas Venezuela on Feb. 7-17, 1982. At that time it was also noted that a continuously variable spectrum of properties can be found not only geographically between deposits but also laterally and vertically within a given petroleum occurrence. Accordingly, the proposed definition employs essentially an arbitrary demarcation between bitumen and heavy crudes, when the materials are compared on the basis of these physical properties alone.
Additional distinctions between bitumen and conventional heavy crude oil may be made on the basis of their chemical compositions. Relative to most heavy crudes, bitumen has a large asphaltene component. Asphaltenes are complex, polynuclear hydrocarbons which are insoluble in n-pentane and/or n-heptane. Due to their substantial asphaltene content, bitumens exhibit a high carbon/hydrogen ratio. For the preparation of transportation fuels, it is generally necessary to reduce the carbon/hydrogen ratio by addition of hydrogen through catalytic hydrogenation. Bitumen typically also contains significant amounts of sulfur, nitrogen and metals as contaminants, often substantially more than most conventional heavy crudes.
The predominating mineral component of the tar sands bitumen material as mined is in most cases silica in the form of quartz sand or diatomite of particle sizes generally greater than about 10 microns, usually greater than about 44 microns (325 mesh) and up to about 10 mesh. The material as mined is surrounded by bitumen in quantities of perhaps about 5 to over 20 weight percent of the total composition. In addition, tar sands generally also contain finer mineral matter of particle size less than about 44 microns and larger than 2 microns, sometimes referred to as silt, and material finer than 2 microns, sometimes referred to as clay. Reference to U.S. Pat. No. 3,811,614. The fine mineral matter may include one or more of calcite, silica, rutile (TiO.sub.2), calcium sulfate and mica as well as clay minerals such as kaolinites or smectite. The fine mineral matter generally has an appreciable content of alkaline earth material present as calcium carbonate and/or calcium sulfate. Fine mineral matter (i.e., mineral particles finer than about 10 microns) is generally present in quantities of from about 1 to about 50 weight percent of the total composition.
The bitumen as found in naturally occurring tar sands is not of great economic value in its crude form. Such bitumen, however, may be upgraded to hydrocarbons of lower molecular weight, in particular to hydrocarbons which are liquids at room temperature. Extensive recovery of tar sand oil has not been seriously considered until relatively recently, primarily because of the expense of known recovery and upgrading methods in relation to the cost of preparing the same products from crude petroleum. The rising costs of crude petroleum production and the depletion of known petroleum reserves, however, have made an efficient and economical process for the treatment of such tar sand increasingly desirable. The vastness of the known deposits has encouraged many people to look at these raw materials as a potential source for filling energy and chemical feedstock needs in a world of depleting conventional crude oil sources.
Several methods have been developed for purifying tar sands to provide bitumen concentrates that can be used as feedstock for further upgrading to produce useful products. The principal purification technique which has been applied to tar sands in order to concentrate bitumen therefrom is extraction. In one type of extraction, commonly known as the "hot water" process, advantage is taken of the fact that tar sands produce a bituminous slurry when mulled with hot water and sodium hydroxide. This slurry divides into two components upon further dilution with hot water in a settling zone. A bituminous froth rises to the surface of the water and is withdrawn for further concentration of bitumen, while essentially bitumen-free sand is discarded as a downward flowing aqueous tailings stream.
Another known beneficiation process for recovery of bitumen from tar sand is known as the "cold water" process. This process comprises the following steps: grinding the ore in the presence of water and a dispersant; flotation with fuel oil; dilution of the bitumen concentrate with solvent; and separation of beneficiated bitumen from the sand/water residue. This process for the preparation of a bitumen concentrate avoids the requirement of large quantities of heat needed to raise the temperature of the water in the process described in the preceding paragraph. In the first stage of preparation, the tar sands as mined are crushed, for example in a gyratory crusher, to form a coarse ore stock pile. Through the use of cone crushers, rod mills and/or ball mills, the latter possibly in closed circuit with cyclones, a product which is approximately 80% below 150 microns may be prepared. Water and a major portion of the conditioning and flotation reagents used in the process are then added to form a slurry. A variety of materials may be added to the crushed tar sand ore prior to conditioning and flotation. Fuel oil or other solvent, in quantities of about 5 lbs. per ton, may be added at this stage. Sodium carbonate (up to about 10 lb/ton) and/or sodium silicate (up to about 5 lb/ton) may also be employed. The slurry is then passed to one or more conditioning tanks. Some conditioning may also be accomplished merely by the flow of slurry through pipes over extended distances. The sized and conditioned slurry is then fed to a flotation circuit, comprising one or more flotation trains. Each of these trains comprises a rougher/scavenger unit and a single- or multiple-stage cleaner circuit utilizing flotation cells. A typical retention time in the flotation cells is on the order of 15 minutes. Tails from the scavenger cell are passed to thickeners, to which lime or another suitable flocculant, in an amount of about 5 lb/ton, is added. Overflow water from the thickener is recycled back into the circuit. A tailings slurry at about 50-60% by weight solids is discharged into a tailings pond. Concentrates from the last stage of the flotation process, containing approximately 25 percent by weight bitumen, are then suitable for further concentration, for example, by solvent upgrading. Unfortunately, fine mineral matter floats with the bitumen concentrate and is not effectively removed by flotation.
As currently practiced, bitumen concentrate from the flotation process is transferred to a mixing vessel where it is combined with at least one part, and generally several parts, of liquid solvent per part of bitumen. While the exact amount and composition of the solvent is not critical, it has been suggested that for maximum effectiveness the solvent should contain about 20% aromatics. Heretofore, the solvent has been almost entirely recovered in subsequent steps. It is possible to use the same fuel oil for solvent upgrading as is used in the flotation process. The diluted bitumen is pumped into settling or holding tanks, where the remaining water and sands begin to settle out.
The final stage of the solvent upgrading process comprises the solvent or diluent recovery stage. This may be a distillation tower or other mechanism which is used to separate solvent and flotation oils for recycling to upstream stages of the extraction process. Depending on the nature of the charge to the solvent upgrading step and the intended use for the concentrated bitumen, additional separation steps, such as dehydration or centrifugation may be necessary.
In yet another type of extraction process, tar sand agglomerates are contacted with a suitable solvent such as a gas-oil boiling range fraction to produce a solution of bitumen and gas-oil. This solution is separated from the sand and then passed to a conventional hydrocarbon conversion unit.
Treatment of tar sands by these beneficiation techniques in order to separate an enriched bitumen stream from the sand is a substantial component of the recovery costs, above those mining the crude ore. These processes generally provide products which contain residual fine mineral matter even after repeated treatments. The contents of fine mineral matter (particles finer than 10 microns) in beneficiated bitumen concentrates typically range from 2500 ppm to 20 weight %, usually below 2 weight %, based on the weight of the bitumen. Since at least a portion of the fine mineral particles forms a stable emulsion with water it cannot be readily removed from the bitumen recovered from tar sands. All of the known processes for preparing bitumen concentrates from tar sands provide products containing at least some residual fine mineral solids. Because of the viscosity of the bitumen and the chemical constitution of the components thereof, it has not been possible to remove the very fine mineral solids from the bitumen by conventional methods, such as hydroclone separators or conventional filtration means. This mineral matter, particularly the mineral matter of finest particle size, introduces additional complications in hydrogen addition treatments, as noted below. The water, which may be present in amount up to 15% based on the weight of the bitumen, also causes problems in downstream upgrading processes. For example, water causes foaming to take place in cokers.
Selective mining has been employed heretofore to minimize the content of mineral solids in bitumen concentrates obtained from tar sands. Tar sands bitumens containing high levels of fine mineral solids are generally not exploited. On the other hand, it is technically feasible but expensive to remove high levels of coarser mineral particles (sands) from bitumen.
In addition to mineral solids, bitumen concentrates generally contain high levels of sulfur, nitrogen, metals and other contaminants. These contaminants have also heretofore presented major problems in the subsequent use of the recovered product.
Retort methods similar to those used in the pyrolysis or thermal cracking of oil shale have been proposed for the recovery of bitumen from tar sands. The raw tar sand is contacted with spend sand and fluidized by reactor off gas at temperatures above about 900.degree. F. Volatile products are flashed off while coke is deposited through thermal cracking. The coke is burned off in a separate unit at 1200.degree.-1400.degree. F. and the sand recirculated. Substantial amounts of spent sand, for example 5-10 parts per part of raw tar sand, are needed for the process. This makes necessary a very large retort volume per barrel recoverable oil. Serious waste heat and handling problems also arise with this process, making it of little interest commercially.
Once the bitumen has been recovered (concentrated) from the tar sands, two primary bitumen upgrading routes are available: carbon rejection and hydrogen addition. Carbon rejection upgrades bitumen by removing asphaltenes, and is examplified by conventional solvent deasphalting, delayed coking and fluid coking processes. Various modifications of the basic coking process have also been proposed. For example, U.S. Pat. No. 2,905,595 describes a process in which tar sands are subjected to a coking process to produce coker gas, gasoline and gas oil and a coke-laden sand stream. The coke-laden sand is contacted with an oxygen-containing gas, such as air, to effect combustion of coke deposited on the sand grains, thereby producing a clean hot sand stream which is recirculated into the process. According to the preferred method described in this patent, coke-laden sands are burned and heated in a specially-designed gas lift furnace. The coke-laden sand is suspended in a plurality of parallel vertical burning zones and recycled through a furnace zone surrounding these vertical tubes. This process produces a distillate product directly. The method essentially employs a recirculated stream of hot solids simultaneously to vaporize, coke and crack the hydrocarbon fraction.
U.S. Pat. No. 3,320,152 described a process in which tar sand agglomerates are introduced into a feed preparation zone and admixed with relatively hot contact material in order to drive off water and reduce the viscosity of hydrocarbon material, thereby providing a fluidizable mixture of sand particles and hydrocarbons. A portion of the fluidizable mixture is passed through a pressure-developing zone and then introduced into a reaction zone containing a fluidized bed of solid particulate material. This reaction zone is maintained under conditions to carry out thermal coking of hydrocarbon material.
U.S. Pat. No. 4,082,646 describes a modified direct coking process in which the combustion stage is divided into two sequential operations. In the first operation, coke solids produced in a reaction are introduced into a coke burning zone where they are contacted with combustion air and the minimum amount of supplementary fuel, if any needed to burn substantially all the coke. Part of the solids is discarded while the remainder, required for heating the coking reaction zone, is introduced into a fuel burner zone. Here the major portion of the supplemental fuel required to maintain heat balance is combined with air or oxygen to heat further the clean solids until their heat content is sufficient to meet the requirements of the coking reaction zone.
Carbon rejection alone cannot deal with the bitumen upgrading job in a cost effective manner. This is because an extraordinarily high amount of either a coke byproduct or an asphalt byproduct is produced. These by-products necessarily contain high contents of sulfur, metals and ash rendering the coke or asphalt relatively valueless. Moreover, the production of unnecessary coke or asphalt markedly reduces the yield of lighter, more valuable liquid hydrocarbons. This yield consideration is of particular importance with respect to tar sand processing, where mining represents roughly 80% of the total operating costs. Thus, an increase in usable fuel yields from each ton of ore can result in disproportionately large overall cost savings.
U.S. Pat. No. 4,161,442 describes a process in which high temperature solids comprising silica are combined with tar sands in a thermal stripping operation restricted not materially to exceed incipient cracking of the petroleum materials. The operating temperature is limited to within the range of 600.degree. F. to 850.degree. F., and preferably below 800.degree. F. An oily residue deposited on the sand is used to generate fuel gas by heating to a temperature above 1500.degree. F. with addition of steam or air. Since the fluid distillation is operated to minimize cracking, the concentration of residual oil material on the sand is relatively high, and only those components which vaporize below the temperatures of incipient cracking are removed. This process provides only minimal amounts of desirable liquid hydrocarbon products because of the low process temperatures employed.
An alternative route for the upgrading of bitumen is hydrogen addition. When hydrogen addition is used alone as the upgrading route, the large amount of hydrogen required to prepare useful products from the hydrogen-deficient asphaltene molecules raises the cost of the fuel produced thereby to unacceptable levels. Moreover, nickel, vanadium and asphaltenes interfere with the hydrogenation and conversion catalysts, shortening run lengths and requiring a more frequent replacement of catalyst. Any fines present in the hydrogen addition feedstock not only block the active sites of the hydrogenation catalyst, thereby reducing its activity, but also lead to the formation over time of obstructions in the flow path of the feedstock through the catalyst bed. This in turn leads to the development of large pressure gradients in the system, ultimately resulting in its shutdown. Combinations of prior art carbon rejection and hydrogen addition processes would only serve to compound the most undesirable characteristics of each.
Another method of deriving useful hydrocarbon products from heavier precursors such as bitumen is the method of catalytic cracking. During the 1930's, the process constituted a major advance over the earlier techniques for increasing pressure to charge catalytic cracking units with heavier crudes and products such as bitumen. Two very effective restraints have limited the extent to which this has been practical: the coke precursor content and the metals, especially heavy metals, content of the feed. As these values rise, the capacity and efficiency of the catalytic cracker are adversely affected.
Polynuclear aromatics, such as asphaltenes, tend to break down during the catalytic cracking process to form coke. This coke deposits on the active surface of the catalyst, thereby reducing its activity level. In general, the coke-forming tendency or coke precursor content of a material can be ascertained by determining the weight percent of carbon remaining after a sample of the material has been pyrolyzed. This value is accepted in the industry as a measure of the extent to which a given feedstock tends to form coke when treated in a catalytic cracker. One method for making this evaluation is the Conradson Carbon Test. When a comparison of catalytic cracking feedstocks is made, a higher Conradson Carbon number (CC) reflects an increase in the portion of the charge converted to "coke" deposited on the catalyst. The Conradson Carbon test has been adopted as an American National Standard and is described in ASTM Method D189. Another generally accepted method for evaluating coke precursor content is the Ramsbottom Carbon test, as described in ASTM Method 524. The Conradson Carbon test, however, is the preferred method for sample that are not mobile below 90.degree. C., such as bitumens.
It has bee conventional to burn off the inactivating coke with air to "regenerate" the active surfaces, after which treatment the catalyst is returned to cyclic fashion to the reaction stage for contact with and conversion of addition feedstock. The heat generated in the burning regeneration stage is recovered and used, at least in part, to supply heat for vaporization of the feedstock and for the cracking reaction.
The regeneration stage generally operates under a maximum temperature limitation in order to avoid heat damage to the catalyst. When feedstock with a high CC content is processed, a larger amount of the feedstock in weight percent is deposited as coke on the catalyst than would be the case with low CC feedstock. When this catalyst is regenerated, the additional coke leads to high temperatures in the regenerator. At these higher temperatures, a number of problems arise. The circulation rate of the catalyst is reduced, often resulting in lower conversion rates. Incomplete regeneration of the catalyst may also occur, reducing its catalytic activity. Finally, if the temperature of the regenerator is sufficiently high, an inactivation of the catalyst takes place. There is thus a practical limit to the amount of coke which can be burned per unit time.
As CC of the charge stock is increased, cokeburning capacity becomes the limiting factor, often requiring a reduction in the rate of charge to the unit. Moreover, part of the charge is diverted to an undesired reaction product, thereby reducing the efficiency of the process. Since bitumen comprises to a great extent hydrogen-deficient, high molecular weight hydrocarbons such as asphaltenes, a direct catalytic cracking of bitumen would clearly be a highly inefficient method for upgrading for this reason alone. This is confirmed by Bunger et al., "Catalytic Cracking of Asphalt Ridge Bitumen", Advances in Chemistry Series, No. 179, "Refining of Synthetic Crudes", p. 67 (1979). These authors report an inhibited rate of catalytic cracking, low octane numbers for the gasoline produced and substantially higher coke make than experienced presently for commercial gas-oil cracking.
An additional drawback to direct catalytic cracking of bitumen is the metals content of the feed. Most bitumens contain heavy metals such as nickel and vanadium. These metals are deposited almost quantitatively on a catalytic cracking catalyst as the molecules in which they occur are broken down. The deposits of these metals build up over repeated cracking cycles to levels which become troublesome. Some of these metals also unfavorably alter the chemical composition of catalysts. For example, vanadium tends to form fluxes with certain components of common FCC catalysts, lowering their melting point to a degree that sintering begins at FCC operating temperatures with resultant loss of catalytic activity.
The heavy metals present in bitumens recovered from tar sand are also potent catalysts for the production of coke and hydrogen from the cracking feedstock. The lowest boiling fractions of the cracked product--butane and lighter--are processed through fractionation equipment to recover components of value greater than as fuel for the furnaces. This fraction comprises primarily of propane, butane and olefins of like carbon number. Hydrogen, being incondensable in the "gas plant", occupies space as a gas in the compression and fractionation train. As the metals level of the charge stock is increased, hydrogen production becomes the limiting factor, often requiring a reduction in the rate of charge to the unit. Moreover, since bitumen is already hydrogen deficient, the generation of additional hydrogen therefrom would be a serious problem.
The sodium content of bitumen also present problems for a conventional catalytic cracking system. Sodium reacts with a zeolite catalyst to produce the inactive form of zeolite. The product bitumen generally contains at least about 1% water, with significant amounts of sodium compounds dissolved therein. These sodium compounds comprise primarily sodium carbonate and sodium hydroxide, which are conventionally used as conditioning agents in the upgrading of tar sands. These compounds are deposited on the catalyst as the bitumen is subjected to catalytic cracking, and can lead to a substantial deactivation of the catalytic cracking catalyst over time, requiring its replacement. Sodium, like vanadium, also tends to form fluxes with certain FCC catalyst components.